The present invention relates to a method for producing three-dimensional structures using an etching process.
Methods, and the equipment for carrying them out, are known for etching silicon or for producing microlenses from a silicon substrate. German Patent No. 42 41 045 discusses an inductive coupled plasma source used for silicon deep-etching at very high etching rates, and discusses using an inductive high frequency excitation in order to liberate fluorine radicals from a fluorine supplying etching gas and (CF2)x radicals from a passivating gas delivering teflon-forming monomers. The plasma source here generates a high-density plasma having a relatively high density of ions (1010-1011 cmxe2x88x923) of low energy, while etching and passivating gases for etching trenches into a masked silicon wafer are used alternatingly in an empirically determined time sequence.
It has further already been proposed in unpublished German Patent Application No. DE 197 363 70.9 to influence the material-dependent etching removal rate as a function of time during etching of a silicon wafer by means of so-called xe2x80x9cparameter-rampingxe2x80x9d, so as in this manner to reduce the profile deviations of etched structures in the silicon wafer.
However, the named methods can be used only for making an etching process, which is isotropic per se, locally anisotropic, and thus, for example, for etching trenches of defined depth and good profile fidelity. However, they do not offer the possibility of transforming a three-dimensional original shape, positioned on a substrate, in a controlled manner into a three-dimensional target shape having a defined surface shape.
Particularly in the production of microlenses, whereby at first an optical surface is generated by melted-on polymers, such as a photoresist, by reason of their surface tension, and this optical surface is then transferred by a dry etching process into an inorganic substrate material, such as silicon, for IR optics, the problem exists of transferring this surface shape of the melted-on polymers during the etching process into the substrate and of simultaneously changing the surface shape of the original polymer in a targeted way to a desired surface shape of the microlens in the substrate material.
During melting on of the polymers, disregarding gravitational forces, with appropriately small lenses and free adjustment of the contact angle, in general, spherical surfaces are created, as are also used for macroscopic lenses. However, in lenses having greater diameters, i.e. several mm, if the influence of gravitational forces can no longer be ignored, aspherical surfaces of the melted-on polymer materials are created, as is well known, which, having circular base planes, are cylindrically symmetrical, and, when projected into the xz-plane of the coordinate system can be described by conical functions hU(x), with hU(x) given by             h      U        ⁡          (      x      )        =            H      1        -          xe2x80x83        ⁢                            R          1                -                                            R              1              2                        -                                          (                                  1                  +                                      K                    1                                                  )                            ⁢                              x                2                                                                1        +                  K          1                    
having a radius of curvature R1, a lens height H1 and a conical constant K1xe2x89xa00. After the etching process in the known etching methods, these aspherical surfaces can be seen again transferred to the substrate, and here, moreover, it frequently happens that the conical constant K1 in the melted-on polymer as original shape changes to conical constant K2 during etching and thus during forming of the original shape into the substrate.
Thus is created a likewise aspherical microlens in the substrate, having, however, different optical properties than would have been expected on the basis of the shape of the melted-on polymer. A sufficient correction or avoidance of this aspherical surface of the microlenses is often not possible using known static etching processes, even when they have different etching removal rates for the substrate material and for the polymer.
As a rule, this is due to a very high conical constant of the original shape of the polymer, a volume change of the original shape by the etching process, or a time-dependent variation of the etching parameters during the etching process, such as by heating of the substrate when cooling is faulty.
The relationship of the etching removal rate of the substrate material to that of the original shape material, by the way, is designated as the selectivity S. Known static etching processes, therefore, have constant selectivities.
Furthermore, it is already known that, in the case of photoresist lenses having a diameter of several mm, at first conical constants with K1 less than xe2x88x92100 set in. From these, and at a selectivity of S=4 which is beneficial for the surface quality of the lenses, known static etching processes can be used only to produce microlenses in a silicon substrate which have a conical constant K2 less than xe2x88x925. In particular, a desired setting of K2=0, that is, a spherical lens, is not possible.
The procedure according to the invention has the advantage, compared to the related art, that it is suitable for producing three-dimensional structures such as, for instance, spherical microlenses of mostly any size at all, in a substrate such as silicon, there being initially in a plurality of places particularly on the substrate at least one original shape having a known original surface shape. Because the etching removal rates for the substrate material and the original shape material differ definitively and, at the same time, they can be selectively set by setting apparatus etching parameters as a function of time, so that the selectivity of the etching process changes definitively with time, therefore the original surface shape can be transferred during the etching process to a desired and predefined target surface shape. This is carried out advantageously in that the original shape is removed during the etching process, and in its place a target shape is structured from the substrate.
Apart from a few apparatus influences which have to be determined by test etchings, the calculation of the required time-related change of the etching removal rates is made before beginning the etching, exclusively using the known surface shape of the original shape and that of the target shape, so that a steady control of the etching process, costly empirical experiments for reaching a desired target shape, and a continuous determination of the surface shape reached up to this point of the target shape as a function of etching time, are not necessary.
To facilitate the calculation, it is very advantageous if the original surface shape can be described at least approximately by an initial function, hU in particular, explicitly known before start of the etching process, and if the target surface shape can be described at least approximately by a target function, HS in particular, explicitly known before start of the etching process.
The apparatus setting of at least one etching parameter as a function of time during the etching process takes place advantageously and in a known manner, for example, by the change of the flow of at least one of the etching gases being used, the concentration and/or composition of the etching gas being used, the process pressure in the plasma etching chamber, the voltage applied to the substrate, the substrate temperature, or the coil voltage of ICP plasma equipment. A computer controlled and operated control unit is especially suitable for setting the individual parameters. In this connection, one may use especially the advantageous property that the etching removal rates also depend on the material.
Starting from an original shape present on the substrate, the method according to the present invention permits in a simple way structuring a desired target shape from the original shape.
Thus, it is simply and advantageously possible to convert an aspherical lens having a circular base plane as the original shape, which is made, for example, of melted-on polymer such as a photoresist, which is present on a silicon substrate or another semiconductor substrate, and whose surface can be described by a conical function, into a spherical microlens structured from the silicon substrate during the etching process.
Thus, with the aid of the method according to the present invention, a clearly improved optical quality of microlenses is achieved, and particularly when working with larger microlenses the development process is clearly shortened, since extensive tests and etchings, while varying the etching parameters, become unnecessary.
Advantageously, however, the method according to the present invention is neither tied to special materials or compositions of the original shape nor to the target shape to be achieved, as long as suitable etching processes known per se are available.
Then, too, the possible surface shapes of the original and the target shapes for the method according to the present invention advantageously encompass multiple geometrical formations, and are not limited to shapes such as aspherical microlenses which have, up to now, been particularly important in practice.
The method according to the present invention is especially suitable for producing three-dimensional structures having a circular base area and a surface shape cylindrically symmetrical to the z-axis of the coordinate system, since, in this case, in the substrate plane defined by the substrate, etching with equal removal rates or equal selectivity takes place at all locations, which only changes as a function of time.
However, in principle it is also possible, using the etching method according to the present invention, to change at least one of the etching parameters or the selectivity as a function of the location on the substrate surface and/or as a function of time, so that, for example, the etching removal rates for original and target shapes can be not only a function of time, but also of the location coordinates x and y. This continuation of the method according to the present invention, which only requires a greater computing effort for the calculation of the changes to be set in the etching parameters, is as yet, however, not of practical significance, since the corresponding etching equipment is still not available. For etching parameters variable as a function of the spatial coordinates and of etching time, the temperature of the substrate or the substrate voltage for example, come into consideration.
In particular, using the method according to the present invention, it is advantageously possible, starting from an original shape having a circular base area and a surface shape described by a first conical function hU having a radius of curvature R1, a conical constant K1 and height H1, to convert this, via a time-related variation of at least one etching parameter calculated before the etching process, into a target shape in a substrate whose surface shape is described by a second conical function HS having a radius of curvature R2, conical constant K2 and height H2. In the especially relevant case where K2=0, in this manner one advantageously attains, in particular, a spherical target shape, while in the case of K1xe2x89xa0K2 one can convert in targeted manner the first conical function, which describes the surface shape of the original shape, into the second conical function which describes the surface shape of the target shape. A particularly simple case of this method arises advantageously in the case which is especially important in practice, that the heights H1 and H2 and/or the radii of curvature R1 and R2 are equal. Thus, the method according to the present invention permits advantageously producing a spherical target shape from an aspherical original shape.
The method according to the present invention further proceeds especially simply if, during the etching process, the etching removal rate of a material such as the substrate material, is held at least substantially constant, and only the etching removal rate of the other material, such as the original shape material, is changed as a function of time via the etching parameters. This reduces the computing effort before etching, increases the reliability and precision of the respectively set etching parameters and the reproducibility of the etching results.
In order to be able to take into account etching equipment-connected deviations in the etching removal rate and the selectivity arising during the etching process, which are not at first taken into consideration in the calculation, and which lead to deviation in the actually achieved surface shape of the desired shape and the surface shape of the target shape taken into account in the calculation, advantageously a test etching can at first be undertaken in addition, the time-related change of the etching parameters yielded by the calculation in view of the surface shape of the original shape (described by the function hU) and the desired surface shape of the target shape (described by the function HS) being used at first. After that, a surface shape often sets in at first which deviates slightly from the desired surface shape of the (K target shape and which is described by a function HS,Test. In order to be able to at least offset this deviation in the first place, after that, advantageously, in all further etching processes an advance calculation of the change of the etching parameters for converting the original shape into the target shape having a newly defined function is undertaken,
hS,new=2hSxe2x88x92hS,Test
which takes the place of the formerly used function hS.